A typical bottom hole assembly is depicted in FIG. 1. With reference to FIGS. 1-4 and 6A, the bottom hole assembly 10 may include elements such as a drill bit 12, a main housing 13, a rotary steerable system (RSS) 14 associated with a hydraulic block 16, a drive shaft 18, a shall lubricating block 60 and other components necessary for securing the drive shaft to the rotating drill string located above the shaft lubricating block.
With reference to FIGS. 2-4, hydraulic block 16 is mounted to main housing 13. Hydraulic block 16 contains a hydraulic pump 20, a hydraulic fluid reservoir 22 containing hydraulic fluid and appropriate passageways, not shown, for conveying hydraulic fluid to actuate the steering arms of RSS 14. Additionally, to provide for pressure compensation versus the ambient downhole pressure, hydraulic block 16 includes a compensation piston 26 located in a fluid passageway 28. On one side of compensation piston 26, fluid passageway 28 communicates with the exterior of hydraulic block 16 through port 77 and provides fluid communication for exterior drilling mud to exert ambient downhole pressure on compensation piston 26. On the other side of compensation piston 26, fluid passageway 28 communicates with hydraulic fluid reservoir 22. A spring 34 located on the drilling mud side of compensation piston 26 within fluid passageway 28 exerts an additional pressure on compensation piston 26. The additional pressure is sufficient to ensure that compensation piston 26 maintains hydraulic fluid reservoir 22 at a pressure greater than ambient pressure. Typically, spring 34 is selected to maintain hydraulic fluid reservoir 22 at a pressure of about 30 psi greater than the ambient drilling mud pressure. Spring rates for spring 34 may range from 5 psi to 50 psi.
During drilling operations, a delay in the operation of RSS 14 can result in misdirected wellbore. The combination of ambient drilling mud pressure and spring pressure acts on the hydraulic fluid within hydraulic fluid reservoir 22 to maintain a pressure greater than the ambient annulus pressure. Accordingly, performance of RSS 14 depends upon the action of drilling mud pressure and spring pressure on the hydraulic fluid within reservoir 22 to ensure that an adequate supply of hydraulic fluid is available at hydraulic pump 20.
Unfortunately, this configuration allows for the introduction of mud particles and other wellbore debris into fluid passageway 28. Overtime, the debris will reduce the reaction time of compensation piston 26 due to increased friction within passageway 28. Eventually, the accumulation of mud debris on the whole side of compensation piston 26 will freeze compensation piston 26. As a result, actuation of RSS 14 steering arms will be delayed due to an inadequate supply of hydraulic fluid resulting in a poorly drilled wellbore.
As depicted in FIGS. 2-4, hydraulic pump 20 is located in a separate passageway 36 from compensation piston 26. Hydraulic pump 20 divides passageway 36 into downhole and uphole regions. Located in the uphole region of passageway 36 is a floating piston 38. Floating piston 38 acts to balance pressure between hydraulic block 16 and shaft lubricating block 60. Finally, a plug 42, located uphole of floating piston 38, seals passageway 36. As depicted in FIG. 4, a fluid passageway 44 and port 32 provide fluid communication between hydraulic fluid reservoir 22 and the uphole area between floating piston 38 and plug 42. Thus, clean hydraulic fluid applies pressure to the uphole side of floating piston 38 while shaft oil from shaft lubricating block 60 passes through port 85 to apply pressure to the downhole side of floating piston 38.
As depicted in FIGS. 1-3, main housing 13 supports shaft lubricating block 60 at a position uphole of hydraulic block 16. Main housing 13 includes first and second bearings 62, 64 which provide supplemental support to drive shaft 18. Bearings 62 and 64 are located within oil reservoir 65. Thus, bearings 62, 64 are submerged in oil.
For proper operation, oil reservoir 65 must be maintained at a pressure greater than ambient pressure. To provide for this necessity, shaft lubricating block 60 includes passageways 74 and 76. Passageways 74 and 76 are divided into downhole and uphole regions by pistons 78, 80. A port 77 provides fluid communication between the downhole regions of fluid passageways 74 and 76 and the exterior of shaft lubricating block 60. As depicted in FIGS. 2-4, the uphole region of fluid passageways 74, 76 contains shaft oil and the downhole region contains drilling mud. Thus, drilling mud applies ambient pressure to the downhole side of pistons 78, 80. Typically, the springs 84, 86 associated with pistons 78, 80 are selected to ensure that the oil in oil reservoir 65 is maintained at about 30 psi above ambient borehole pressure. Spring rates for springs 84, 86 may range from 5 psi to 50 psi. In the prior art configuration of FIGS. 1-8, springs 84 and 86 do not provide any pressure compensation benefit to hydraulic block 16. Rather, in the prior art configuration compensation pressures generated by springs 84, 86 are balanced against the compensation pressure generated by spring 34 of hydraulic block 16 by floating piston 38.
As depicted in FIG. 3A, shaft oil flows through port 82 into oil reservoir 65 and across first and second bearings 62, 64 to port 85. Port 85 provides fluid communication with passageway 36 of hydraulic block 16. Thus, shaft oil passes from shaft lubricating block 60, through oil reservoir 65 of main housing 13 and into hydraulic block 16 where it contacts the downhole side of floating piston 38. As discussed above, fluid passageway 44 and port 32 provide fluid communication between hydraulic fluid reservoir 22 and the uphole area between floating piston 38 and plug 42.
The described configuration balances the pressures experienced by hydraulic block 16 and shaft lubricating block 60. However, overtime the lubricating fluid of shaft lubricating block 60 becomes contaminated with wear particles produced by rotating drive shaft 18. These contaminants will increase friction experienced by floating piston 38 and will lead to delayed movement on the part of floating piston 38 creating an imbalance of pressure between the two operating blocks. This imbalance of pressure could lead to leakage of lubricating fluid from shat lubricating block 60 into hydraulic block 16 contaminating the hydraulic fluid and disrupting steering operations. Additionally, bearings 62, 64 impede the flow of shaft oil from shaft lubricating block 60 to hydraulic block 16 as port 82 is located uphole of bearing 64 while port 85 is located downhole of bearing 62. Thus, shaft oil experiences a constricted flow path as it crosses each bearing. Thus, this configuration does not efficiently transfer hydraulic pressure from shaft lubricating block 60 to floating piston 38. Accordingly, the effective pressure experienced by floating piston 38 is less than expected which can result in a delay of steering arm deployment by the RSS. Any delay in steering arm deployment will increase steering error during drilling operations and increase operational costs.
The following disclosure describes an improved hydraulic block and improved shaft lubricating block. The improvements preclude the contamination of passageway 28 housing the compensation piston 26 with debris carried by the drilling mud. Additionally, the improvements provide for elimination of floating piston 38 from passageway 36.